1. Field of the Invention
The present invention relates generally to circuit design, and, more particularly, to the design of passive mixer circuits having a positive voltage gain and a balanced DC mixing function.
2. Description of the Related Art
Typical radio frequency (RF) communications systems utilize mixers to shift, or translate, the frequency of an input signal (fIN) by mixing it with a Local Oscillator (LO) signal of a known frequency (fm). The frequency of the resulting output signal (fOUT) is either the sum or difference of fIN and fm (e.g. fOUT=|fIN+fm| or fOUT=|fIN−fm|), depending on whether the mixer is used for downconversion or upconversion. Mixers are typically used in both receivers and transmitters, oftentimes to perform downward frequency translation, which is commonly referred to as downconversion. Downconversion is useful in a receiver because it reduces the frequency of received signals, enabling any subsequent signal processing to be carried out at baseband or at an intermediate frequency (IF) where it is more tractable. Mixers are also used to perform upward frequency translation, commonly referred to as upconversion. Upconversion is useful in a transmitter because it enables shifting the modulated low-frequency data to higher frequencies where wireless transmission is more tractable.
Mixers can be passive and/or active mixer circuits, both types of mixer circuits being fairly common in current RF transceivers. Active type and passive type mixer circuits both come with their own advantages and disadvantages. Active mixer circuits consume DC bias current, while passive mixer circuits do not. One important advantage of active mixers, such as a Gilbert Cell, for example, is the relatively large conversion gain they typically provide, easing noise requirements and reducing the power consumption of subsequent stages. Active mixers, however, typically require substantial bias current and operating voltage headroom, and the active circuitry is also prone to generating (1/f) noise, which can be problematic in Low-IF and direct conversion receiver topologies. Furthermore, the downward scaling of voltage supplies and feature-sizes in the IC fabrication processes can exacerbate the voltage headroom and noise issues present in active mixers. Passive switching mixers, which typically use transistors configured to function as switches, can be used as an alternative to active mixers. Passive switching mixers offer inherently low power consumption because they do not consume DC bias current. Furthermore, they typically do not require voltage headroom to operate, generate significantly lower (1/f) noise than active mixers, and derive the output signal energy predominantly from the energy of the input signal. It should be noted, however, that in practice, energy from other undesired sources may also appear at the output, for example feed through from the LO signal and thermal noise. However, such undesired energy will affect most all implementations, including those featuring active mixers, thus the benefit of low power consumption when using passive mixers therefore remain substantial.
Overall, passive (switching) mixers can achieve linearity and noise performance levels comparable with those of active mixers, while consuming less power. Furthermore, the performance and power consumption of passive FET switching mixers can also improve with the scaling of field effect transistor (FET) technology (e.g. metal oxide semiconductor—MOS) fabrication processes (e.g., implemented with MOSFET devices). However, one notable disadvantage of passive mixers is their lack of conversion gain. As a matter of fact, an inherent property of passive frequency translation is actually signal power attenuation. Passive mixers can exhibit a conversion loss of 2 dB to 6 dB, which can offset the power consumption advantage of the passive mixer, as subsequent stages of a receiver (i.e. stages following the mixer) must achieve lower noise levels to maintain a given signal-to-noise ratio (SNR) with an attenuated signal level. Hence, subsequent stages typically need to consume more power to achieve a given system noise figure. It would therefore be advantageous to design mixers having the power saving advantages of passive mixers as well as the ability to achieve voltage conversion gain.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.